Generally described, a combined cycle system uses the combination of a gas turbine and a steam turbine to produce electrical power. Specifically, a gas turbine cycle may be operatively combined with a steam turbine cycle by way of a heat recovery steam generator and the like. The heat recovery steam generator is a heat exchanger that allows feed water for the steam generation process to be heated by the hot combustion gases of the gas turbine exhaust. The primary efficiency of the combined cycle system arrangement is the utilization of the otherwise “wasted” heat of the gas turbine engine exhaust. Specifically, the efficiency of the heat recovery steam generator focuses on the heat transfer between the gas turbine combustion gases (“the hot side”) and the feed water and the steam (“the cold side”). The goal is to use as much of the heat and pressure of the gas turbine combustion gases as possible to provide useful work.
Although a combined cycle system is efficient, there are numerous types of parasitic losses involved in overall system operation. During hot ambient conditions at partial load, for example, the temperature of the exhaust gases from the gas turbine also may be higher. These higher temperatures generally may require more water to be sprayed into a reheat attemperator so as to control the temperature of the steam exiting the reheaters. This additional water/steam, however, bypasses the high pressure turbine without producing useful work so as to reduce overall system performance. The extra amount of water sprayed during partial load performance may be a multiple of the reheat spray at base loads such that any reduction may have a significant impact on overall performance.
There is thus a desire for an improved combined cycle power plant or other type of gas turbine system with reduced parasitic losses and operating losses. Preferably, the amount of water required for the reheat attemperator and similar components may be controlled during partial load operations and/or during other conditions so as to improve overall performance and efficiency.